A type of nuclear reactor known as a Pressurized Water Reactor uses water, which is maintained at a high pressure and which is caused to circulate through the reactor primary coolant system, as a means of removing the heat generated by the nuclear fuel in the reactor core. The filling of the primary coolant system in this type of reactor is not without its problems. During the filling operation, as the water level rises above the level of the coolant system circulation pipes, the air remaining in the upper portions of the reactor vessel, the vessel head, and the control rod drive mechanism becomes trapped preventing complete filling of the coolant system. In order to complete the filling operation, the closure devices used for the control rod drive mechanisms must be vented to allow the trapped air to escape. Similarly, when the water level in the coolant system must be reduced as, for example, at refueling time, the closure devices must be vented to allow air to enter the control rod drive mechanisms so that the cooling water can be rapidly drained from the reactor. In order to accomplish this venting operation, vent valves have typically been incorporated into the closure devices. Through repeated use and because of inherent design problems, after a period of time these vent valves may not seat properly and can require excessive maintenance, repair, or replacement resulting in increased exposure of maintenance personnel to nuclear radiation due to the time required to perform such functions. In addition, in most instances these valves require the use of a special venting tool to operate same.
Because of this, it has become desirable to develop a closure assembly for a control rod drive mechanism that has venting means incorporated therein and which retains proper sealing capabilities after repeated use and does not require a special venting tool to operate same.